In recent years, a rapid growth of the electronics industry and the communications industry involving a variety of information and communications technology, including mobile communications, has led to increasing demands for greater compactness in electronic devices. To meet these demands, mobile electronics and communication terminals, such as laptops, netbooks, tablet PCs, cellphones, smartphones, PDAs, digital cameras, and camcorders, have been made widely available. As a result, there is also an increasing interest in the development of batteries, which are the driving power source for these devices.
Furthermore, with the development of electric vehicles, such as hydrogen electric vehicles or hybrid electric vehicles, and fuel cell vehicles, much attention is focused on developing a battery of high performance, high capacity, high density, high power, and high stability. Also, developing a battery having fast charging and discharging rates has become a major issue.
A battery, which is an apparatus for converting chemical energy into electrical energy, is classified as a primary battery, secondary battery, fuel cell, solar cell, or the like.
Among these, primary batteries, such as manganese batteries, alkaline batteries, and mercury batteries, produce energy through an irreversible reaction. Hence, even though primary batteries have a high capacity, they have a disadvantage of being non-recyclable, and thus, various problems including energy inefficiency, environmental pollution, etc. are inherent in this type of battery.
Examples of secondary batteries include lead-acid batteries, nickel-metal hydride batteries, nickel-cadmium batteries, lithium-ion batteries, lithium polymer batteries, lithium metal batteries, etc. Secondary batteries are chemical batteries which are capable of repeating charging and discharging cycles using reversible conversions between chemical energy and electrical energy. Because secondary batteries operate by means of reversible reactions, they have an advantage of being recyclable and eco-friendly.
A secondary battery has four basic components: a positive electrode, a negative electrode, a separator, and an electrolyte.
The positive electrode and negative electrode are electrodes in which energy conversion and storage occur through oxidation-reduction, and these electrodes respectively have a positive potential and a negative potential. The separator is placed between the positive electrode and the negative electrode to maintain electrical isolation, and also provides a channel for electric charge migration. In addition, the electrolyte serves as a medium for delivering the electric charge.
Each of the electrodes comprises a respective electrode active material. The active materials used in a lithium secondary battery, which currently is the secondary battery of most interest, are provided below.
For the most part, materials that allow the intercalation of lithium ions are used as a positive electrode active material. Examples include oxides, such as lithium cobalt oxides (LixCoO2), lithium nickel oxides (LixNiO2), lithium nickel cobalt oxides (Lix(NiCo)O2), lithium nickel cobalt manganese oxides (Lix(NiCoMn)O2), spinel-type lithium manganese oxides (LixMn2O4), and manganese dioxide (MnO2); olivine-type or NASICON-type phosphates, such as lithium iron phosphates (LixFePO4) and lithium manganese phosphates (LixMnPO4); silicates; sulfates; and polymeric materials.
As a negative electrode active material, lithium metal or its alloys, or compounds that allow the intercalation of lithium ions, may be used. Examples include polymeric materials and carbon materials, and more specifically, graphite types such as artificial or natural graphite; and carbon types, such as non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), carbon nanotube (CNT), carbon nanofiber (CNF), and carbon nanowall (CNW).
The above electrode may generally be manufactured by painting an electrode current collector with an electrode active material slurry and drying the slurry and thus forming an electrode active material layer. The electrode active material slurry generally comprises an electrode active material, a conductive agent, a binder and other additives such as a dispersion medium. More specifically, the electrode may be manufactured by weighing each ingredient that forms the electrode active material slurry and mixing the ingredients; coating the electrode current collector with the mixture and drying the mixture; and thereafter pressing the mixture.
As mentioned earlier, an electrode is generally structured to have an electrode current collector and an electrode active material layer formed on the electrode current collector, or additionally a middle layer, comprising a binder and a conductive agent, between the electrode current collector and the electrode active material layer in order to enhance the performance of the electrode.
However, electrodes having a middle layer as described above may be short-circuited as network paths between conductive agents become disconnected due to the expansion of the binder occurring above a certain temperature; and consequently, a battery comprising such an electrode may have degraded performance. Therefore, there is a need for developing a stable high-power electrode.